I. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for adaptively estimating the channel conditions of a wireless communication channel.
II. Description of the Related Art
In a wireless radiotelephone communication system, many users communicate over a wireless channel. Communication over the wireless channel can be one of a variety of multiple access techniques that allow a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
The CDMA technique has many advantages. An exemplary CDMA system is described in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeatersxe2x80x9d, issued Feb. 13, 1990, assigned to the assignee of the present invention, and incorporated herein by reference. An exemplary CDMA system is further described in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSystem And Method For Generating Signal Waveforms In A CDMA Cellular Telephone Systemxe2x80x9d, issued Apr. 7, 1992, assigned to the assignee of the present invention, and incorporated herein by reference.
In each of the above patents, the use of a forward-link (base station to mobile station) pilot signal is disclosed. In a typical CDMA wireless communication system, such as that described in EIA/TIA IS-95, the pilot signal is a xe2x80x9cbeaconxe2x80x9d transmitting a constant zero symbol and spread with the same pseudonoise (PN) sequences used by the traffic bearing signals. The pilot signal is typically covered with the all-zero Walsh sequence. During initial system acquisition, the mobile station searches through PN offsets to locate a base station""s pilot signal. Once it has acquired the pilot signal, it can then derive a stable phase and magnitude reference for coherent demodulation, such as that described in U.S. Pat. No. 5,764,687 entitled xe2x80x9cMobile Demodulator Architecture For A Spread Spectrum Multiple Access Communication System,xe2x80x9d issued Jun. 9, 1998, assigned to the assignee of the present invention, and incorporated herein by reference.
A functional block diagram of a typical prior art forward link data formatter as used by a CDMA base station is shown in FIG. 1. Data source 102 may be, for example, a variable rate vocoder such as that described in U.S. Pat. No. 5,657,420, entitled xe2x80x9cVariable Rate Vocoderxe2x80x9d, issued Aug. 8, 1997, assigned to the assignee of the present invention and incorporated herein by reference. Data source 102 generates traffic channel information in the form of frames of digital data. CRC and tail bit generator 104 calculates and appends cyclic redundancy check (CRC) bits and tail bits to the frames generated by data source 102. The frame is then provided to encoder 106, which provides forward error correction coding, such as convolutional encoding, upon the frame as is known in the art. The encoded symbols are provided to repetition generator 120, which repeats the reordered symbols to provide the appropriate modulation symbol rate. The repeated symbols are then provided to interleaver 108, which re-orders the symbols in accordance with a predetermined interleaver format. The repeated, interleaved symbol stream is then covered with one of a set of orthogonal Walsh sequences in traffic Walsh coverer 122, and gain adjusted in gain element 124. It should be understood that other forward link data formatters are also known in the art. For example, it is well known that the repetition generator 120 may be placed after the interleaver 108.
Pilot signal generator 128 generates a pilot signal, which may be a sequence of all ones. The pilot signal is then covered with the all-one Walsh sequence and combined with the output of gain element 124 in combiner 136. The combined pilot channel and traffic channel data (which may be plus or minus ones) is then spread in PN spreader 138 using a complex PN code generated by PN generator 140, and then transmitted by radio frequency transmitter 142 over antenna 144. A similar forward link data formatter is disclosed in co-pending U.S. patent application Ser. No. 08/886,604, entitled xe2x80x9cHigh Data Rate CDMA Wireless Communication Systemxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
Other data formatting techniques also exist. For example, in the cdma2000 reverse link, the pilot signal is time-multiplexed with power control commands. Additionally, in W-CDMA, the forward link uses dedicated pilot signals that are time-multiplexed with other information.
FIG. 2 illustrates a functional block diagram of a typical prior art data demodulator for use in a CDMA mobile station. Receiver 202 receives and downconverts the signals transmitted by transmitter 142 of FIG. 1. The digital baseband output of receiver 202 is despread in PN despreader 204 using the complex PN code generated PN generator 206, which is the same complex PN code as that generated by PN generator 140 of FIG. 1.
The despread signal is then Walsh uncovered in traffic channel Walsh uncoverer 208 using the same Walsh sequence as that of the traffic channel Walsh coverer 122 of FIG. 1. The Walsh-uncovered chips are then accumulated into Walsh symbols in Walsh chip summer 210 and provided as a traffic channel signal to dot product circuit 212. In some applications, an additional delay element (not shown) is introduced between Walsh chip summer 210 and dot product circuit 212 to account for delays introduced by pilot filter 216. However, if pilot filter 216 is a causal filter, such a delay element (not shown) is not necessary. The dot product circuit is also known as a xe2x80x9cconjugate productxe2x80x9d circuit. It performs the operation expressed mathematically by one of the following equivalent forms:  less than a,b greater than =axc2x7b=ab*, where b* is the complex conjugate of b.
The despread signal is also provided to Walsh chip summer 214 where they are accumulated into Walsh symbols and provided to pilot filter 216 as pilot channel symbols. Note that since the pilot channel is covered with the all-one Walsh sequence in Walsh coverer 134 of FIG. 1, a vacuous operation, the corresponding uncoverer is also vacuous in operation. However, in the general case, the pilot signal may be uncovered using any same Walsh sequence as is used to cover it. The pilot filter 216 serves to reject the noise in the pilot symbols, providing a phase and scale reference for the dot product circuit 212.
Once per symbol, the dot product circuit 212 computes the component of the traffic channel signal in phase with the pilot channel signal generated by the pilot filter 216. As described in U.S. Pat. No. 5,506,865, entitled xe2x80x9cPilot Carrier Dot Product Circuitxe2x80x9d, issued Apr. 9, 1996, assigned to the assignee of the present invention and incorporated herein by reference, the dot product adjusts both the received signal""s phase and scale as needed for coherent demodulation.
The symbols output from dot product circuit 212 are de-interleaved in de-interleaver 218, using the same format used by interleaver 108 of FIG. 1. The de-interleaved symbols are then decoded in decoder 220 according to the error correcting codes employed by encoder 106 of FIG. 1. The resulting decoded symbols are analyzed on a frame-by-frame basis by CRC Check 222 to ensure that the frame was properly decoded. If the frame was properly decoded, then that decoded frame is forwarded for further processing. CRC Check 222 typically would examine the CRC portion of the frame, but may also use other frame quality indications such as Yamamoto metrics.
A typical pilot filter 216 is implemented as an equal-weight finite impulse response (FIR) filter with all defining parameters (e.g., weighting, window width, window center) remaining constant regardless of the channel conditions. Alternately, an exponential decay infinite impulse response (IIR) filter having fixed parameters (e.g., decay time constant, scaling) may be used. In other words, the designer of a typical prior art pilot filter 216 will choose a static filter design that performs adequately for a given energy per bit to noise density ratio (Eb/N0) under most channel conditions of interest, but not optimally over the entire range of conditions.
As a mobile station (e.g., a cellular telephone, PCS telephone or other wireless remote communication terminal) moves through the terrestrial environment, the signals it transmits and receives will experience various types of fading. The mobile environment is usually characterized by fading that can be either Rician or Rayleigh in nature. Other types of fading are also possible. The fading characteristic in the typical channel signal is caused by the signal being reflected from many different features of the physical environment, thus it is called multipath fading. At the UHF frequency bands usually employed for mobile radio communications, including those of cellular mobile telephone systems, significant phase differences in signals traveling on different paths may occur. The possibility for both constructive and destructive summation of the signals may result, with on occasion deep fades occurring.
Multipath channel fading is a function very sensitive to the physical position of the mobile unit. A small change in position of the mobile unit changes the physical delays of all the signal propagation paths, which further results in a different phase for each path. Thus, the motion of the mobile unit through the environment can result in a rapid fading process. For example, in the 850 MHz cellular radio frequency band, this fading can typically be as fast as one fade per second for every mile per hour of vehicle speed. Fading this severe can be extremely disruptive to signals in the terrestrial channel resulting in poor communication quality, particularly as the speed of the mobile station increases beyond 150 km/hr.
As previously stated, the typical fixed-parameter pilot filter 216 is not optimized for such a broad range of channel conditions. It is typically designed to work adequately at speeds from stationary to about 120 km/hr, or about as fast as a mobile station might be expected to travel in a motor vehicle on the highway. However, since the fading characteristics of the channel are vastly different as between a slow-moving mobile station and a fast-moving mobile station, the typical fixed-parameter pilot filter 216 cannot be optimized for both extremes. Typically, this forces the designer to design a pilot filter 216 that works well only when the mobile station is stationary or moves slower than about 150 km/hr, and works poorly beyond 150 km/hr. As transportation such as bullet trains and airplanes exceeds this speed, it is unlikely that the user of a mobile station will be able to obtain reliable communications. Even when operational, the Signal-to-Noise Ratio (SNR) (or in other words, the Eb/N0) of the communication link must be kept at a high enough level to be reliable in these severe fading conditions. Increasing the Eb/N0 of the communication link decreases the total capacity of the wireless system, particularly in a CDMA system where one transmitter""s transmissions comprise interference to all other transmitters in the same CDMA frequency band. As a result, the designer of a fixed-parameter pilot filter 216 generally adopts an unfavorable compromise in selecting the filter parameters when faced with such a broad range of channel conditions.
Thus, there is a need for a more optimal pilot filtering method and apparatus, particularly in the wireless communication environment, that avoids these shortcomings in the prior art.
The present invention is a novel and improved method and circuit for adaptively estimating channel conditions of a pilot channel in a wireless communication system. The method includes estimating channel statistics of the pilot channel, and adaptively filtering the pilot channel in response to the estimated channel statistics. The estimation is performed by filtering a channel signal derived from the pilot channel, or any channel bearing ambiguous data with the ambiguity removed after detection or decoding. This determines an estimated channel mean and an estimated channel covariance. In order to perform the adaptive filtering, the present invention partitions the pilot channel into one or more time slots and weights each time slot according to the estimated channel statistics. Thus, an advantage of the present invention is that it automatically and continually updates the pilot filter parameters in order to optimize the pilot filter performance over a broad range of channel conditions.
In one embodiment of the present invention, the channel signal is filtered in one or more infinite impulse response (IIR) filters to determine the channel statistics. In a another embodiment, the channel signal is filtered in a combination of IIR and finite impulse response (FIR) filters to determine the channel statistics.
In an embodiment applicable to the cdma20000 system wherein the pilot channel comprises a pilot signal portion having a known sign and a power control bit portion having an unknown sign. The method of the present invention may also include determining the sign of the power control bit portion, correcting the sign of the power control bit portion, and then combining the sign-corrected power control bit portion with the pilot signal portion to generate the channel signal from which the channel statistics are estimated by the channel statistics estimator. In yet another embodiment, the method includes determining the sign of the traffic signal and generating the channel signal in response to the traffic signal.
In yet another embodiment applicable to a cdma2000 system, wherein the pilot channel comprises a sequence of power control groups, each power control group having a pilot signal portion having a known sign and a power control bit portion having an unknown sign, the time slots have a duration substantially equal to the duration of the pilot signal portion.
A circuit for performing the method of the present invention is also described herein.